1. Field of the Invention
The invention relates to a rotary electric machine control system and method, and more particularly to a rotary electric machine control system and method that switch a control mode of a rotary electric machine.
2. Description of Related Art
There is a method of controlling a rotary electric machine to be used as a motor or an electric generator, in which an appropriate one of control modes: a sine-wave pulse width modulation (PWM) control mode, an over-modulation control mode, and a rectangular-wave control mode, is selectively used. The over-modulation control mode is sometimes referred to as over-modulation PWM control mode.
For example, Japanese Patent Application Publication No. 2010-81663 (JP-2010-81663 A) describes a rotary electric machine control system that is designed to switch a control mode between a sine-wave current control mode, an over-modulation current control mode, and a rectangular-wave voltage phase control mode. This patent literature also describes that when a maximum efficiency characteristic line along which the rotary electric machine can be operated at its maximum efficiency is defined on a d-q plane defined by a d-axis and a q-axis that are used for vector control of the rotary electric machine, a switching line serving as a reference for switching from the rectangular-wave voltage phase control mode to the over-modulation current control mode is set on the smaller angle side of the maximum efficiency characteristic line. This patent literature says that chattering in control during mode switching can be avoided by setting the switching line on the smaller angle side.
However, when the switching line serving as a reference for switching from the rectangular-wave control mode to the over-modulation control mode is set on the smaller angle side of the maximum efficiency characteristic line on the d-q plane, the switching from the rectangular-wave control mode to the over-modulation control mode will be slow if the number of revolutions per unit time of the rotary electric machine rapidly drops. As a result, a voltage to be applied to the rotary electric machine may remain high even though the number of revolutions of the rotary electric machine has decreased. The rapid drop of the number of revolutions of the rotary electric machine can occur, for example, when the rotary electric machine is mounted on a vehicle for driving wheels, and the wheels shift from a slip state to a grip state. For example, when this vehicle runs on a wavy road, slip and grip may possibly be repeated alternately. The rapid drop of the number of revolutions of the rotary electric machine due to the occurrence of sudden grip may also occur when the vehicle runs on a projection on a road surface such as a cat's eye. In this case, the voltage applied to the rotary electric machine becomes excessively higher than a normally required voltage, and hence excessive phase current may flow through stator coils of the respective phases of the rotary electric machine. Thus, there is a room for improvement in effective prevention of possible breakdown of equipment caused by such excessive phase current.
Although the problem that may occur when the control mode is switched over from the rectangular-wave control mode to the over-modulation control mode has been described above, the problem of excessive current flowing though the rotary electric machine at the time of a rapid drop of the number of revolutions of the rotary electric machine may also occur when the control mode is switched over from the over-modulation control mode to the sine-wave PWM control mode. Specifically, a modulation degree is used for switching between the over-modulation control mode and the sine-wave PWM control mode. The modulation degree is a ratio of an effective value of line-to-line voltage that is a voltage applied to the rotary electric machine to a system voltage VH that is a direct current (DC) voltage of an inverter. If the modulation degree used for switching from the control over-modulation control mode to the sine-wave PWM control mode is too small, the switching from the over-modulation control mode to the sine-wave PWM control mode will become slow when the number of revolutions per unit time of the rotary electric machine rapidly drops, and as a result, the voltage applied to the rotary electric machine may remain high even though the number of revolutions of the rotary electric machine has been decreased. For example, when the rotary electric machine is mounted on a vehicle for driving wheels, and the wheels shifts from a slip state to a grip state, a voltage that is excessively higher than normally required may be applied to the rotary electric machine, and hence excessive phase current may flow through stator coils of the respective phases of the rotary electric machine. Thus, there is a room for improvement in effective prevention of possible breakdown of equipment caused by such excessive phase current.